1. Field of the Invention
The present invention relates to a dimensionally stable metal hydride composition. In particular, the present invention relates to a metal hydride composition that retains the finely-powdered particles that result from repeated hydrogen absorption/desorption cycles in a dimensionally stable matrix, and a method for making such a composition. The United States Government has fights in this invention pursuant to Contract No. DE-AC09-895R18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Hydride-forming materials that rapidly absorb/desorb large amounts of hydrogen under controlled temperature and pressure conditions are used in heat pumps, batteries, and fuel cells, and for hydrogen storage, pumping, compressing, purifying and isotope separating.
Most elemental metals and many metal alloys are capable of reacting with hydrogen to form a hydride. Known hydride-formers (loosely termed hydrides) include pure metals (Mg, Ti, V, Nb, Pt, Pd, and so forth), alloys, and hydride compositions consisting of a hydridable material mixed with a non-hydridable ballast or matrix. To maximize their surface area and absorption/desorption efficiency, hydrides and hydride compositions are usually supplied in the form of small-grained particles or pellets.
As a metal absorbs hydrogen, it expands and internal stresses cause it to fracture and break apart into smaller pieces, a process called decrepitation. The particle size is reduced with each absorption/desorption cycle until, eventually, the particles disintegrate into a submicron-sized powder ("fines"). The hydride fines compact, conduct heat poorly and do not readily allow hydrogen to permeate so less hydrogen is absorbed during each successive cycle. If the powder becomes entrained in the gas stream, it can migrate and contaminate downstream piping and equipment. Even if filters are used to contain the powder, the fine mesh required for such small particles is easily clogged.
A number of attempts have been made to alleviate these problems. For example, Helversen (U.S. Pat. No. 4,249,654) coats metal hydrides onto the surface of diatomaceous earth, rare earth, porous ceramic, or glass fiber. Baker, et al. (U.S. Pat. No. 4,600,525) adds a lubricant such as TEFLON beads to a powdered hydrogen sorbent alloy to serve as a flow aid. Flow aids prevent compaction by allowing rearrangement and movement of the powders with respect to each other and with respect to the side walls of the container.
More commonly, hydrides are combined with stable, non-hydridable matrix materials to form compositions that are better able to withstand repeated absorption/desorption cycles than hydrides alone. Metal hydrides may be plated with a dissimilar metal (Ishikawa, et al., U.S. Pat. No. 4,717,629) or mixed with non-hydridable matrix metals (Ron, et al., U.S. Pat. No. 4,507,263; Goodell, et al., U.S. Pat. No. 4,6817,650; Bernauer, et al., U.S. Pat. No. 4,310,601; Steyert, et al., U.S. Pat. No. 4,360,569). Ovshinsky, et al. (U.S. Pat. No. 4,431,561) incorporates a light weight rare earth element into a host matrix to increase the numerical density of catalytically active sites for dissociating hydrogen molecules. Roberts, et al. (U.S. Pat. No. 3,803,082) increases the thermal stability of aluminum hydrides by incorporating small amounts of an alkyl or aryl substituted silicol into the hydride lattice. Other hydride compositions are formed by combining a metal hydride with a matrix material such as plastic powder (Steyert, et al., U.S. Pat. No. 4,360,569), compounds such as tin oxide and aluminum oxide (Leppard, U.S. Pat. No. 4,459,270), or polymers (Bernstein, et al., U.S. Pat. No. 4,433,063; Buhl, et al., U.S. Pat. No. 4,110,425; Blytas, U.S. Pat. No. 4,036,944).
Dimensionally stable hydrogen-absorbing compositions and methods for making those compositions are disclosed in two commonly assigned and recently filed patent applications. Application Ser. No. 07/933,152 (Palladium/Kieselguhr Composition and Method), filed 8/21/92, describes a composition made by depositing palladium onto a porous substrate such as kieselguhr from a concentrated solution of tetra-amine palladium (II) nitrate. Application Ser. No. 07/952,93 1 (Dimensionally Stable Metallic Hydride Composition), filed 09/29/92, describes a composition made by subjecting a metal hydride to one or more hydrogen absorption/ desorption cycles to disintegrate the hydride particles. The particles are partly oxidized, then blended with a ballast metal and kieselguhr to form a uniform mixture. The mixture is compressed into pellets and calcined.
Many of these compositions have a relatively low hydride content, and therefore a lower hydrogen-absorbing capability than a pure hydride of the same volume. Furthermore, the expansion of the hydride particles during hydrogen absorption creates very high stresses in the immediate vicinity of the particles. It has proved to be difficult to develop a composition having sufficient strength to accommodate the expansion of the hydride without fracturing, and a porosity high enough to allow the transfer of hydrogen to and from the hydride particles, but low enough to contain the fines produced by repeated cycling.
There is a need for a dimensionally stable hydride composition that can reversibly absorb large amounts of hydrogen. The matrix should have sufficient porosity for the hydrogen to permeate the structure. It should maintain its hydrogen-absorbing capacity over a large number of absorption/desorption cycles, and, preferably, retain substantially all of the hydride fines created during absorption/desorption.